Inflammatory destruction of target organs in autoimmune diseases, including rheumatoid arthritis, begins with the emigration of effector cells from the microcirculation. Suppression of inflammatory leukocyte extravasation could provide protection against joint damage in rheumatoid arthritis. However, it is not very well understood how immune cells gain access to the joints, and how the process of leukocyte recruitment to the synovium is regulated. CD44 and leukocyte (L)-selectin are two major adhesion molecules that mediate rolling, the very first step of leukocyte interaction with the endothelium of postcapillary venules within target tissues. Our previous work indicated a pro-inflammatory role for CD44 in arthritis. We demonstrated that treatment with CD44-specific antibodies ameliorated joint inflammation, and that disruption (knockout) of the CD44 gene increased resistance to disease in arthritis-susceptible mice. CD44-null mice that develop disease show an altered expression of L-selectin, suggesting the possibility of cooperation between CD44 and L-selectin in directing lymphocyte traffic to the joints. Recently, we have introduced intravital (in vivo) videomicroscopy to monitor the kinetics of leukocyte recruitment into inflammatory sites, created by local injection of tumor necrosis factor alpha, using wild type mice and mice lacking CD44 or L-selectin or both. Preliminary in vivo studies on this inflammation model suggest that CD44 is involved in at least two steps of leukocyte recruitment, rolling and firm adhesion. In CD44-deficient mice leukocytes roll in a L-selectin dependent manner and exhibit reduced adhesion and transendothelial migration. Removal of both CD44 and L-selectin (double gene knockout) results in a nearly complete blockade of leukocyte extravasation, tn the studies proposed here, in vivo videomicroscopy will be applied to the mouse knee joint. Leukocyte traffic to synovium will be monitored during the course of arthritis in two models of immune-mediated inflammatory joint disease: antigen-induced arthritis (AIA) and proteoglycan-induced arthritis (PGIA). The contribution of CD44 and L-selectin to inflammatory leukocyte recruitment in arthritis will be determined in wild-type mice, and in mice deficient in CD44 or L-selectin, or both. As an additional approach, we will use antibodies and other inhibitors against CD44, L-selectin, or both, to antagonize cell rolling, and thus reduce inflammation in established PGIA. The direct effects of antibodies and other antagonists on leukocyte recruitment will be examined in the synovial microcirculation of arthritic knee joints in wild type mice. The studies proposed here should provide insight into leukocyte-endothelial cell interactions in inflamed joints, and determine the requirement for CD44 and L-selectin in arthritis. Information gained from this work will also help identify and test therapeutic reagents that, by antagonizing leukocyte extravasation, could protect joint tissues from autoimmune inflammatory attacks, even when the immune response has already been generated.